Thanks for all your help, Punnam.

One last thing, I want to change the formula to this for larger data in Actual effort sheet:

SUMPRODUCT(('Actual Effort'!$B:$B=$E11)*('Actual Effort'!$E:$E=$F11)*((TEXT('Actual Effort'!$F:$F,"m-YY")=TEXT(Summary!G$3,"m-yy"))*('Actual Effort'!$C:$C)))

Unfortunately (again), I'm getting this error #Value